One easy solution is to deploy a VMware Virtual Appliance. These are preconfigured virtual machines that run within VMware's virtualization software. WIth both VMware Server and VMware Viewer available for free (though not open source), this gives you a way to get up and running with someone else's notion of the perfect software setup quickly. With over 700 virtual appliances available (some free, some commercial), the selection here is your best bet for finding more obscure packages. Sometimes that leads to having to wade through and compare choices, though: for example, there are a dozen different Asterisk implementations here, with various features and configurations.

For a more tightly targeted - and more expensive - alternative, take a look at JumpBox. JumpBox offers preconfigured virtual machines for (at the moment) 20 applications, including Joomla, WordPress, Trac, and SugarCRM. Unlike VMware virtual appliances, JumpBoxes are useable in a wide variety of virtualization products: VMware, Microsoft Virtual Server, Parallels and Virtual Iron are all on the list. However, licensing a single JumpBox will cost you $40, more if you also want paid support incidents. There's also a new "JumpBox Open" subscription which gives you a 12-month subscription to everything they put out for a year, at a cost of $200.

For a free, and non-virtualized approach, check out BitNami. These folks supply installers that work atop your existing Linux, Mac, or Windows operating system, isolating an entire application's stack in its own folder. They cover a variety of software including WordPress, Subversion, Joomla, phpBB, and more. Of special interest are their infrastructure stacks - for example, you can install their RubyStack and have everything you need to start Rails development after just a few clicks.
Of course, there are pros and cons to using pre-built application stacks. On the plus side, you can get up and running quickly. On the minus side, you may have less understanding of what you've done than you would have if you built everything from scratch. But as a way to lower adoption barriers for open source software, they're hard to beat.